Diamond synthesized by chemical vapor deposition (“CVD”) has many unique and outstanding properties that make it an ideal material for a broad range of scientific and technological applications. A number of methods for diamond CVD are reported which utilize various gas mixtures and energy sources for dissociating the gas mixture. Such methods include the use of high temperature electrons in various kinds of plasma, high solid surfaces on hot filaments, and high temperature gases in combustion flames to dissociate molecules such as hydrogen, oxygen, halogen, hydrocarbon, and other carbon containing gases. Typically, a diamond crystal or film is grown on a substrate, which is usually maintained at a temperature much lower than that of electrons in the plasma, the heated surface of a hot filament, or the combustion flame. As a result, a super equilibrium of atomic hydrogen is developed near the diamond growing surface of the substrate.
Atomic hydrogen is believed to be crucial in the diamond CVD process. It is theorized that atomic hydrogen is effective in stabilizing the diamond growing surface and promoting diamond growth at a CVD temperature and pressure that otherwise thermodynamically favor graphite growth. Consistently, the reported diamond CVD processes involve the use of hydrogen gas or hydrogen containing molecules. The most typical diamond CVD process utilizes a precursor comprising of methane gas diluted by 94-99% hydrogen. With these CVD processes, the super equilibrium of atomic hydrogen can be achieved at a varied percentage of molecular hydrogen in the gas mixture. However, these CVD processes depend on the effectiveness of the dissociation process in generating atomic hydrogen.
Using a high power density microwave plasma to deposit diamond in a precursor comprising of a mixture of methane and hydrogen with less than 50% hydrogen has been reported. Growth of diamond from oxy-acetylene flames utilizes a precursor comprising acetylene and oxygen with a ratio of acetylene to oxygen slightly greater than 1 without additional molecular hydrogen being added. Diamond is deposited in the reducing “inner flame” where atomic hydrogen is a burn product produced by the high temperature flame. In addition to atomic hydrogen, there are plenty of OH radicals present near the diamond growing surface inside the flame.
OH and O radicals can play another role of atomic hydrogen in the diamond growth process. That is, preferential etching of non-diamond carbon, which results in a net deposition of high purity diamond. A small quantity of oxygen (0.5-2%) and/or water vapor (<6%) added to the methane and hydrogen precursor is reported to improve diamond crystallinity and lower the diamond CVD temperature. The quantity, whether small or large, of oxygen and/or water in a precursor or feedstock is a relative term depending on many other process parameters. Diamond has also been grown in a microwave plasma of a precursor comprising an acetone/oxygen mixture with a molecular ratio near 1:1.
Most of the diamond CVD processes involve the use of one or more compressed gases. Typically, such CVD processes utilize a compressed gas precursor comprising 1 vol % methane gas diluted by 99 vol % hydrogen. These gases usually must be precisely controlled by electronic mass flow controllers to ensure the accurate composition in the gas precursor feed.
In U.S. Pat. No. 5,480,686 to Rudder et al. (“Rudder”) a method of diamond growth is disclosed that utilizes a radio frequency (“RF”) plasma in a precursor comprising a mixture of water (more than 40%) and alcohol. No compressed gases are needed for this diamond CVD process. However, water has a low vapor pressure at room temperature, and condensation of water in the cooler part of the reactor manifold may be a concern. Also, water has a high freezing temperature making it easy to freeze at the orifice of a flow controller where liquid vaporizes and enters a low pressure reactor chamber. Buck et al. (“Buck”), (“Microwave CVD of diamond using methanol-rare gas mixtures,” Materials Research Society Symposium Proceedings, Vol. 162, 97-102, 1989.) have grown clusters of diamond crystallites on small (2-4 mm2) silicon substrates that were scratched with a diamond tip or mechanically polished with 3 μm diamond powder by microwave plasma enhanced CVD in pure methanol vapor. Argon gas additive was found necessary for high quality diamond to be deposited in the methanol vapor. When it is fully dissociated and reacted in the plasma, the pure methanol vapor plasma contains a C/O/H composition similar to that of CO/H2 plasma, which has been used for successful deposition of diamond by means of electrical discharges.
In a typical electrical discharge such as a microwave plasma, electrons with an average temperature exceeding 10,000° C. are abundant. These energetic electrons effectively dissociate molecular species and generate a high concentration of radicals necessary for the deposition of diamond and the preferential etching of non-diamond deposits without needing a high temperature filament. Hot filament assisted CVD processes employ solid surfaces at a temperature of about 2,000° C.-2,500° C. to dissociate molecules and generate radicals necessary for diamond deposition. The hot filament temperature is much lower than that of energetic electrons in a plasma. As a consequence, hot-filament CVD of diamond in CO/H2 mixtures has not been successful even though the same gas mixtures have been routinely used for plasma assisted deposition of diamond films.
Nevertheless, the plasma enhanced CVD method is desirable because diamond crystals and films can be deposited on large-area and/or irregularly shaped objects using inexpensive equipment. Thus, there remains a need for an economic method of synthesizing diamond utilizing plasma enhanced CVD.